Fluid jet angular velocity sensors utilizing sensing elements for sensing the speed of rotation are well known in the art. U.S. Pat. No. 3,500,690 to Schuemann, 4,020,700 to Lopiccolo et al., and 3,581,578 to Schuemann, all disclose fluid jet angular velocity sensors having a pair of sensing elements for sensing the speed of rotation about an axis perpendicular to a "plane of sensitivity".
The sensing elements are usually positioned symmetrically about a reference jet axis with each element on opposite sides and at equal distances therefrom. A fluid jet is directed along the reference jet axis from a nozzle which cools the sensing elements in substantially equal proportions in the absence of sensor rotation. Due to the well-known Coriolis effect, the fluid jet impinges nonsymmetrically, i.e., the fluid jet "bends" in the presence of sensor rotation. Because of the well-known characteristic of fluid jets in which the higher velocity fluid particles are concentrated at the center of the jet and the lower velocity particles around its periphery, the sensing elements are cooled in different proportions whenever the fluid jet impinges nonsymmetrically upon the sensing elements.
Parametric studies to evaluate the effects of various geometric design factors on angular rate sensor performance have been undertaken. Such sensors are found to be measurably affected by asymmetries imposed on almost any part of the gas flowpath. Asymmetries associated with the nozzle and sensor plug regions cause especially large output shifts which are often quite flow sensitive. Unequal deflection of sensor wires under jet impact is found to be a particularly troublesome flow dependent asymmetry, especially at high flowrates.
The prior art use of pumps having a Piezoelectric (PZT) diaphragm involves special problems. A source of unrepeatability is changes in the pump impedance due to temperature hysteresis. This causes unrepeatability in flow rate after temperature cycling. The error gradually disappears if the pump is kept at room temperature, but it can take as long as a week for this to occur. This phenomenon is well known for materials with high dielectric constance.
The optimum frequency is difficult to achieve using PZT diaphragms because the PZT pump cannot be driven harder than about 6 volts due to power supply limitations. In addition, with a PZT diaphragm, the deflection is a direct function of voltage and thickness. Changing the thickness is a very time consuming manufacturing operation and it has been found that there is a definite limit on minimum thickness because of manufacturing difficulties. Thus, both minimum frequency and maximum deflection are limited by properties of the PZT material itself.
Various design changes are needed to modify the sensor in order to provide improved flow thereby increasing accuracy.